An offshore floating support moves vertically with the swell. This vertical movement, commonly referred to as heaving, is swell-dependent and is particularly important because it affects the operation of the structures which are both supported by the floating support and connected to the sea floor. These structures, for example, can be drill pipes or pipelines enabling the transport of oil or natural gas. When the floating support heaves, these structures move vertically in relation to the support and, for this reason, it is necessary to equip these structures with telescopic compensation systems making it possible to always compensate for the heaving of the floating support, in order to enable operations at the upper portion of these structures. These compensation systems are very costly, all the more so if the movement compensation to be made is significant, and furthermore, they have technological compensation limits.
The heaving movement is approximately proportional to the swell height, and it is conventionally characterised by the ratio of the heave to the swell height, this ratio first being an approximation, an invariant with respect to the swell height. The heaving movement also depends on the shape of the floatation members, the effect of the swell generating pressure on the walls thereof, of which the cumulative effect on all of the walls always results in a vertical excitation force of the movement. The heaving movement also depends on the swell period, given that the distribution of pressure on a floatation member having a predetermined shape depends on the swell period and on the wavelength thereof (to this end, in deep water, the wavelength of the swell (in meters) corresponds approximately to the square of its period (in seconds) multiplied by 1.56). Finally, the heave also depends on the angle of attack of the swell, i.e., the orientation of the floating support in relation to the direction of swell propagation.
For this reason, the heaving movement of the support, at the geometric centre thereof, (generally the point of connection with the structures connected to the sea floor), is characterised by a heave transfer function, which is the representation of the evolution of the heave/swell height ratio in relation to the swell period.
In order to minimise the heaving movement, in current so-called semi-submersible platforms, each floatation member (typically consisting of a submerged float, the submerged portion of a column supported by the submerged float and supporting the working deck, and half of each of the adjacent submerged connection elements connecting the column-float assembly to the other column-float assemblies) is shaped in such a way that the cumulative effects of the pressures generated by the swell is cancelled out for a predetermined period, conventionally referred to as a balancing period. The heave transfer function for such a platform has a value close to 0 for small periods, increases steadily in order to reach a relative maximum, which is approximately equal to 0.5, drops to 0 again for the balancing period, and then rises again quickly and sharply.
Thus, in the prior art, limitation of the heaving movement is carried out by properly configuring each floatation member of the floating support so that the balancing period that is associated with them is greater than the swell periods ordinarily encountered on the platform operating site. For this reason, for ordinary swells at the site, the heave transfer function will be at most equal to 0.5.
However, this value of 0.5 is relatively significant and involves the use of relatively large compensation systems. Furthermore, the heave transfer function is greater that 0.25 for a significant range of swell periods.
In addition, it is known, e.g., according to U.S. Pat. No. 3,490,406, irrespective of the distance separating the vertical axes passing through the centre of buoyancy of the floatation members, that the heave is particularly abated when the floating support is subjected to a swell the propagation direction of which is that joining the two vertical axes and the period of which is equal to twice the distance separating these two axes.